Patent Application
20130296091
The present invention is related to a variable speed drive, and in particular to a variable speed drive employed in aircraft applications.
Aircraft applications present a variety of unique considerations regarding power distribution and efficiency. Unlike ground-based applications, all power consumed on an aircraft, whether electrical, mechanical, hydraulic, or pneumatic, is derived from power generated by the aircraft engines themselves.
The overall efficiency of the aircraft engines (i.e., amount of fuel consumed) depends on how efficiently the systems on the aircraft utilize the generated power. For example, a number of traditional aircraft systems utilize pneumatic power in the form of bleed air. However, bleed air represents energy loss from the engine, and its use therefore decreases the overall efficiency of the aircraft system. An alternative to pneumatic power derived from bleed air is electric power derived from generators mechanically coupled to the engines. Mechanical power generated by the aircraft engines is converted to electric power by the generators, distributed to a desired load, and converted back to mechanical energy via an electric motor. Traditionally, pneumatic and/or electric power is used to power aircraft systems such as compressors employed in conjunction with environmental control systems (ECS) because of the need to drive the compressors at variable speeds. However, utilizing electric energy requires converting mechanical energy (generated by the rotating aircraft engine) to electric energy for distribution to the motors, and subsequent conversion back to mechanical energy for consumption by the load (i.e., compressor, pump, etc.). The added weight of those components similarly decreases the efficiency of the aircraft engine.
An advanced variable speed drive (AVSD) includes an input shaft connected to receive a mechanical input from an aircraft engine and an output shaft connected to provide a speed-controlled mechanical output. The AVSD includes a first power path having a fixed gear ratio, a second power path having a variable gear ratio, and a differential coupled to combine power received from the first power path and the second power path for provision to the output shaft. A controller modifies the variable gear ratio of the second power path to regulate the output shaft of the AVSD to a desired speed.
The present invention employs an advanced variable speed drive (AVSD) to convert a variable speed mechanical input received from the engine to a controlled, variable speed output. A benefit of the present invention is it allows a variable speed mechanical input to be converted to a controlled, variable speed output without requiring conversion of mechanical engine power to either pneumatic power or electrical power.
Controller 24 receives a speed command spd_cmd that represents the desired speed of output shaft 22. The speed command may be provided by CAC 14, ECS 18 or some other aircraft control system based on operating conditions of the aircraft. Controller 24 also receives a speed feedback signal spd_fdbk from speed sensor 26 that represents the speed of output shaft 22. In response to differences between the speed command signal spd_cmd and the speed feedback signal spd_fdbk, controller 24 provides a control signal to actuator 28 included within AVSD 12 to selectively regulate, in closed loop fashion, the speed of output shaft 22. As described in more detail with respect to
In the embodiment shown in
The output of gear ratio box 30 is split into first and second power paths (i.e., split power path). The first power path includes mechanical gear elements comprising fixed gear ratios. The second power path includes a combination of mechanical and hydraulic gear elements capable of providing variable gear ratios that allow speed to be selectively increased or decreased as desired. Differential 32 is a mechanical device for summing the power provided by the first power path with the power provided by the second power path (i.e., summing the speed of the output provided by the first power path with the speed of the output of the second power path). An exemplary embodiment of differential 32 is illustrated in
The mathematical representation of differential 32 includes second gear ratio box 34 and third gear ratio box 38. Second gear ratio box is represented as (k+1)/1, with the value of k selected based on the application to provide the desired output. Similarly, third gear ratio is represented as 1/−k, with the value of k selected based on the application to provide the desired output. Sum box 36 illustrates mathematically the combination of power/speed provided by the first power path with the power/speed provided by the second power path.
The second power path includes mechanical and hydraulic components, including fourth gear ratio 40, variable displacement unit 42, fixed displacement unit 44, and fifth gear ratio box 46. Fourth gear ratio box 42 mechanically couples the output of gear ratio box 30 with variable displacement unit 42, and applies a gear ratio of 1/k_v. Variable displacement unit 42 converts the received mechanical input to a variable flow of hydraulic power that is communicated to fixed displacement unit 44 (as indicated by the wavy lines connecting variable displacement unit 42 and fixed displacement unit 44). Controller 24 (shown in
As discussed above, differential 32 adds (or subtracts) the power provided by the second power path to the power received from the first power path. The output is applied to sixth gear ratio box 48, represented in this embodiment as 1/k_o, the output of which represents the output speed N_output of output shaft 22. In this embodiment, the value of the variable k_o is set equal to one, although in other embodiments the value of k_o is selected based on the application requirements.
Face clutch disconnect 50 is connected between input shaft 20 and carrier shaft 51, and acts to both mechanically couple input shaft 20 to carrier shaft 51 and to disconnect input shaft 20 from carrier shaft 51 in response to fault conditions to protect internal components of AVSD 12. In the embodiment shown in
Carrier shaft 51 communicates mechanical energy received from input shaft 20 via face clutch 50 to epicyclic differential 58 as part of the first power path. In addition, carrier shaft 51 is connected to communicate mechanical energy to gear 54, defined by gear ratio 1/k_v, which communicates mechanical power to variable displacement piston pump 62 as part of the second power path. In the embodiment shown in
Fixed displacement unit 60 is hydraulically coupled to variable displacement unit 62. The volume of fluid provided by variable displacement unit 62 is provided to fixed displacement unit 60, which converts the received hydraulic power provided by variable displacement piston pump 62 to mechanical energy that is communicated via gear ring 56 defined by gear ratio 1/k_f. In addition, gear ring 56 is connected to supply mechanical power to accessory pumps 68. By adjusting the position of swash plate 64, the volume of fluid pumped by variable displacement unit 62 is varied, and as a result the mechanical energy generated by fixed displacement unit 60 is selectively controlled. In this way, the second power path provides a variable gear ratio that allows the speed of gear 56 to be selectively increased or decreased.
In the embodiment shown in
In the embodiment shown in
In this way, AVSD 12 provides a controlled mechanical output that can be used to selectively vary the speed of attached loads without requiring conversion of mechanical power to pneumatic and/or electric power. A benefit of this arrangement is a reduction in weight and cost associated with typical power conversion systems (e.g., generators for conversion from mechanical to electric conversion, pneumatic motors for conversion from mechanical to pneumatic power).
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
